Madeleine was nearly four when she went missing from her family's holiday apartment in Praia da Luz, Portugal in May 2007. Her parents were having dinner with friends nearby. The photo above is a computer-generated image of how she may look today, aged nine years.

Detective Chief Inspector Andy Redwood, who leads the inquiry said "I believe that there is a possibility that Madeleine is alive."

"The review has given us new thinking, new theories, new evidence and new witnesses to pursue. It is a positive step in our hunt for Madeleine that our understanding of the evidence has enabled us to shift from a review to an investigation," Redwood added.

Redwood said none of the potential suspects were connected to Madeleine's family or friends who were with her parents on holiday at the time. Detectives believe Madeleine was abducted in a criminal act by a stranger.

Police have formally asked the Portuguese authorities for their assistance in the case.

However, the British police said detectives are at an early stage of the new investigation, known as Operation Grange, and arrests are not expected in the immediate future.